Marching to the Beat of Two Different Drummers

Almost all organisms, from bacteria to mammals, have a circadian clock—a mechanism in their cells which keeps them in sync with Earth’s day-and-night cycle. But many organisms follow other rhythms as well: the tides, the months, or the seasons. Although researchers have documented these behaviors, no one has been sure whether these nondaily cycles use the same components as the circadian clock, or if they have their own clocks.

Two papers published today present the first evidence for clocks independent of the circadian one: a sea louse whose swimming patterns sync up with the tides, and a marine worm that matures and spawns in concert with the phases of the moon. The discoveries, by groups working independently, suggest that noncircadian clocks might be common and could explain a variety of biological rhythms.

On the northern coast of Wales lives a tiny crustacean called the speckled sea louse—Eurydice pulchra. Less than a centimeter long, the creature swims and feeds during high tide, which comes every 12.4 hours, but buries itself in the sand during low tide. It also swims more vigorously during daytime high tides than nighttime high tides. The black spots on its shell spread out during the day as a sort of sunscreen but form discrete spots at night. Charalambos Kyriacou, a molecular geneticist at the University of Leicester in the United Kingdom, marine biologist Simon Webster at Bangor University in the United Kingdom, and their colleagues wanted to find out if those two rhythms—the 24-hour cycle of spots and vigorous swimming and the 12.4-hour cycle of activity or resting in the sand—were driven by the same molecular clock.

After painstaking work to pull out the known circadian clock genes (discovered in flies, mice, and other model animals) from the crustacean, the researchers tested whether they could manipulate the two rhythms independently. First, they kept the animals in constant darkness in the lab for more than a month, so that both the swimming and the shell patterns became arrhythmic. By vibrating the animals’ test tubes for 10 minutes every 12.4 hours, the researchers were able to reestablish the animals’ tidal swimming patterns, though there was no difference between day-and-night “high tides.” When they interfered with some of the circadian clock genes, wiping out the animals’ daily patterns, the tidal swimming rhythms were unaffected. “We can completely disrupt the circadian clock, and nothing happens to tidal clock,” Kyriacou says. The creature’s tidal clock must be an independent mechanism, the group reports online today in Current Biology.

The marine worm Platynereis dumerilii gained a bit of notoriety a decade ago when it helped researchers to unravel the evolutionary ancestry of the vertebrate eye. The animals have light-sensitive cells, called photoreceptors, in their brains that are not connected to their eyes, but are surprisingly similar to human photoreceptors. Kristin Tessmar-Raible, a neurobiologist at the University of Vienna who helped lead the molecular genetics studies of the photoreceptors, wondered what the worm actually used them for. In decades-old literature, she found that lunar cycles govern the worms’ maturation and spawning patterns—they spawn on nights around the new moon. “I thought it was a joke,” she says. “It sounds like some fairy tale.” In conversations with marine biologists, however, she learned that patterns of behavior in sync with the moon are not uncommon. Still, it wasn’t clear whether the patterns were connected to circadian clocks.

By exposing animals in the lab to different amounts of dim light at night, Tessmar-Raible and her colleagues were able to artificially shift the worms’ lunar-based rhythms. These rhythms in turn affected the animals’ patterns of day-and-night behavior. But, as they explain online today in Cell Reports, interrupting the animals’ circadian clock with a drug didn’t seem to affect the lunar cycles—demonstrating that the lunar clock is a separate one. (The lunar clock affects the animals’ circadian clock, but not vice versa.)

The work in both papers is “really solid,” says chronobiologist Martha Merrow at Ludwig Maximilians University in Munich, Germany, and definitively answers the question of whether independent, noncircadian clocks exist. The discoveries will prompt other researchers to look for new clocks, she says. “It breathes new life into questions about all these seasonal rhythms, such as the fact that reproduction can be so regulated to specific times of year.” Other still-unexplained cycles such as human menstruation and 17-year cicada emergence might be explained in part by circadian-independent clocks, she says, and a recent paper suggests that human sleep might also be affected by lunar cycles. Tessmar-Raible and her colleagues are now looking for molecular components of the worm’s lunar clock, in part to see if they can find them in other organisms.